High entropy alloy for external components

ABSTRACT

A high entropy alloy with a composition containing between 4 and 9 major alloying elements chosen from the list including Cr, Fe, V, Al, Si, Mn, Mo, Ti and Ni with: 3 major alloying elements which are Cr, Fe and V, each having an atomic concentration include between 20 and 40%, 1 or 2 major alloying elements chosen from Al and Si each having an atomic concentration higher than or equal to 5% with a total concentration of these 2 major alloying elements of less than or equal to 25%, 0, 1, 2, 3 or 4 major alloying elements chosen from Mn, Mo, Ti and Ni, each having an atomic concentration higher than or equal to 5% with a total atomic concentration of these 4 major alloying elements of less than or equal to 35%, the total atomic concentration of the 4 to 9 major alloying elements being higher than or equal to 80%, the remainder being made up of any impurities and/or one or more minor alloying elements, each in an atomic concentration of less than 5%.

FIELD OF THE INVENTION

The present invention concerns a high entropy alloy and an external component for a watch or piece of jewellery made from this alloy.

PRIOR ART

Various alloys are commonly used nowadays for the fabrication of external watch components which are components generally exposed to the external environment that may be in contact with the skin. These are, for example, austenitic stainless steels, titanium alloys or precious metals. Indeed, these alloys have certain important properties for this type of parts, namely high corrosion resistance, high polishability for aesthetic purposes, and no ferromagnetism. In addition to these characteristics, other properties are currently highly sought-after in horology. These characteristics are high biocompatibility, especially by reducing or eliminating potential allergens, such as nickel or cobalt, and a high hardness and scratch resistance. Alloys that meet all these criteria are rare. Precious metals have low hardness (<200 HV in annealed condition). Austenitic stainless steels generally contain nickel and also have limited hardness (<300 HV in annealed condition). Martensitic stainless steels are hard (>600 HV) but ferromagnetic. Finally, titanium alloys, like grade 5 titanium (Ti6Al4V), certainly represent the best compromise among the properties set out above, but they have a particular colour and a hardness (around 350 HV for grade 5 titanium) that is not significantly higher than some austenitic stainless steels, for example. For comparison, amorphous metals, which are also very advantageous for external components, can have a hardness of more than 500 HV. However, very specific implementations are required to obtain amorphous metal components, which further limits their use as external components.

In the field of external timepiece components, there therefore remains a strong interest in obtaining hard, crystalline ferromagnetic alloys (>400 HV in annealed condition), which are corrosion resistant and highly polishable. In this context, high entropy alloys, currently the subject of much research and which form a new class of alloys, are particularly promising. According to the initial definition, alloys containing at least 5 major alloying elements with an atomic fraction of between 5 and 35% were considered high entropy alloys and elements having an atomic fraction of less than 5% were considered minor. These days, it is accepted that alloys containing 4 major alloying elements can be considered high entropy alloys. As regards thermodynamics, the high entropy resulting from mixing various major alloying elements should stabilise solid-solution phases relative to the formation of potentially embrittling intermetallic phases. Consequently, unique properties, seldom seen in traditional alloys based on one or two major alloying elements are obtained. For external timepiece components, obtaining simple solid-solution phases is very advantageous, since it promotes high polishability and high corrosion resistance. Further, the mixture of various elements produces solid-solution hardening. Among single-phase high entropy alloys, high hardnesses have therefore already been demonstrated, particularly for those that have a body-centred cubic structure. These single-phase, body-centred cubic structure, high entropy alloys, such as, for example, NbTiVZr, AlNbTiV, Al0.4Hf0.6NbTaTiZr or Hf0.5Nb0.5Ta0.5Ti1.5Zr, are more specifically intended for high temperature applications, especially for aeronautics. However, they contain many elements that are expensive, very reactive or have high melting temperatures, such as Nb, Zr, Hf, Ta. To facilitate the implementation of external timepiece components, it is important to avoid or limit the quantity of these elements, since high temperature resistance is not a desired property.

SUMMARY OF THE INVENTION

It is an object of the invention to propose a high entropy alloy with a composition specifically adapted to the needs of external components. The present invention more particularly aims to develop an alloy which, after implementation, has a hardness higher than or equal to 400 HV, non-ferromagnetic behaviour and high corrosion resistance.

To this end, the alloy contains 3 major alloying elements which are Cr, Fe and V, each having atomic compositions comprised between 20 and 40%. It also contains Al and/or Si as major alloying element, which has the effect of eliminating the ferromagnetic behaviour of the alloy. These elements each have an atomic concentration higher than or equal to 5% with a total atomic concentration of Al and Si of less than or equal to 25%.

The alloy may also optionally contain one or more major alloying elements chosen from Mn, Mo, Ti and Ni, each in an atomic concentration higher than or equal to 5% with a total atomic concentration of all 4 major alloying elements of less than or equal to 35%. According to the invention, the Ni content is specifically maintained at a value of less than 20% to avoid, during implementation and especially during heat treatments, the formation of undesirable phases which embrittle the material and reduce corrosion resistance. Some grades are also free of Ni to ensure high biocompatibility.

The remainder can be made up of any impurities and/or one or more minor alloying elements, each in an atomic concentration of less than 5%.

Depending on the composition and thermomechanical treatments, the material obtained after implementation has a single-phase with a body-centred cubic structure, which promotes good corrosion resistance and high polishability for a better surface finish or, in the case of multi-phase alloys, a matrix (main phase) with a body-centred cubic structure reinforced by nanoprecipitates. It also has the advantage of having a colour close to that of austenitic stainless steels.

Other advantages will appear from the features set out in the claims, and from the detailed description of the invention illustrated hereinafter with reference to the annexed drawings, given as non-limiting examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a watch case made with the alloy according to the invention.

FIG. 2 represents the diffractogram of an Al6Cr30Fe30Mo5V29 alloy after casting and heat treatment for 3 hours at 1300° C. followed by cooling in a furnace with a mean cooling speed of around 100° C./min.

FIG. 3 represents the hysteresis curve for this same alloy.

DETAILED DESCRIPTION

The present invention relates to high entropy alloys and to their use for external components for watches or pieces of jewellery, especially for components intended to be in contact with the skin. The external component can be a case middle, a case back, a bezel, a pusher, a crown, a bracelet link, a dial, a hand, a how symbol, a clasp, etc. By way of illustration, a watch case 1 made from the alloy according to the invention is represented in FIG. 1.

According to the invention, the alloys include between 4 and 9 major alloying elements. ‘Major alloying elements’ means elements having an atomic concentration higher than or equal to 5%. The alloys include the following 3 major alloying elements: Cr, Fe, V in an atomic concentration comprised between 20 and 40%. They also include 1 or 2 major alloying elements chosen from among Al and Si with a total atomic concentration of these two elements of less than or equal to 25%. They also optionally include one or more major alloying elements chosen from among Mn, Mo, Ti and Ni with a total atomic concentration of these 4 major alloying elements of less than or equal to 35%.

According to the invention, the total atomic concentration of all the aforecited major alloying elements is greater than or equal to 80%. The remainder may, optionally, contain minor alloying elements selected from the list including Si, Mn, Mo, Al, Nb, H, B, C, N, O, Mg, Sc, Ti, Cu, Ni, Zn, Ga, Ge, Sr, Y, Zr, Rh, Pd, Ag, Sn, Sb, Hf, Ta, W, Pt and Au. ‘Minor alloying elements’ means elements having an atomic concentration of less than 5%. The remainder can also contain residual impurities arising from the implementation.

To obtain the alloys according to the invention, any shaping methods can be envisaged. It is possible, in particular, to obtain these alloys by casting, by powder metallurgy processes, by additive manufacturing techniques or by layer deposition technologies. This also includes any thermomechanical treatments (heat treatment, hot deformation, cold deformation) and sintering and hot isostatic pressing steps (HIP).

After shaping and performance of any thermomechanical treatments, the alloys according to the invention mostly have a body-centred cubic structure (BCC), which may be disordered (structure A2, space group Im3m) or ordered (B2 structure, space group Pm3m). In particular, a single-phase microstructure can be obtained at ambient temperature for alloys according to the invention which contain neither Ni, nor Ti as major alloying elements, nor any minor alloying elements, which promotes corrosion resistance and polishability. Nonetheless, depending on the composition and heat treatments carried out, the alloys according to the invention may have a microstructure with a second phase in the form of precipitates, which, in some cases, can improve mechanical properties (hardness, ductility, resistance to deformation, etc.). When the precipitates are small with sizes that may be nanometric and when the matrix has a virtually unchanged composition, i.e. it has a composition that satisfies the definition of alloys according to the invention (multi-element solid-solution phases), the high polishability, high corrosion resistance and absence of ferromagnetism are maintained. In particular, the addition of Ni or of Ni and Ti is particularly interesting, since this makes it possible to obtain very hardening nanoprecipitates.

In short, after implementation, the alloys of the invention have the following properties required for external components: non-ferromagnetic behaviour, hardness higher than or equal to 400 HV, high corrosion resistance, especially with no sign of corrosion after the salt spray test according to ISO standard 9227.

A few examples of alloy compositions which meet all these criteria after fabrication are given in Table 1 below. The alloys were fabricated by arc melting with no other heat treatment. In the table, the atomic fractions have been rounded to the nearest whole number and hardness has been rounded to the nearest ten.

TABLE 1 Hardness Compositions (at. %) (HV10) Al10Fe25Cr40V25 450 Al10Fe40Cr25V25 410 Al10Fe25Cr25V40 500 Al10Fe30Cr30V30 410 Al5Cr30Fe30Mo5V30 480 Al6Cr30Fe30Mo5V29 480 Al5Cr30Fe30Si5V30 460 Al5Cr30Fe30Mn5V30 410 Al13Cr25Fe25Ni12V25 650 Fe25Cr25V25Al10Ni10Ti5 630 Cr31Fe31V31Si7 500

It is observed, in particular, that the addition of nickel makes it possible to significantly increase hardness, owing to the formation of nanoprecipitates of NiAl in the body-centred cubic structure matrix.

After casting and a heat treatment for 3 hours under argon at 1300° C. to homogenise the casting structure, a single phase microstructure is obtained, particularly for alloys containing only major alloying elements without Ni or Ti, such as, for example, for the alloy Al6Cr30Fe30Mo5V29.

An X ray diffraction analysis (Bragg-Brentano configuration) was performed on this alloy and confirmed that a single phase was present with three lines corresponding to the body-centred cubic structure. This diffractogram is represented in FIG. 2.

With regard to the magnetic properties of this alloy, a hysteresis curve was measured at ambient temperature with a vibrating sample magnetometer (magnetisation M according to the applied field H). Although the alloy has a relatively high volume susceptibility (4.8 10⁻³), the alloy exhibits linear behaviour, signature of paramagnetic behaviour, as shown in FIG. 3.

It is also possible to improve the properties, particularly the mechanical properties, by adding some minor alloying elements while maintaining a major phase that meets the definition of alloys according to the invention. It is, for example, possible to add a small amount of boron as minor alloying element. Adding 0.1 at. % of boron to the alloy Al10Cr30Fe30V30 leaves the hardness unchanged relative to the same alloy without boron (410 HV), however, the addition of boron reduces grain growth after heat treatment and thereby improves ductility and polishability. The addition of interstitial atoms such as C, N and 0 as minor alloying elements also makes it possible to increase hardness. 

1. A high entropy alloy with a composition containing between 4 and 9 major alloying elements chosen from the list comprising Cr, Fe, V, Al, Si, Mn, Mo, Ti and Ni with: 3 major alloying elements which are Cr, Fe and V, each having an atomic concentration comprised between 20 and 40%, 1 or 2 major alloying elements chosen from Al and Si each having an atomic concentration higher than or equal to 5% with a total concentration of these 2 major alloying elements of less than or equal to 25%, 0, 1, 2, 3 or 4 major alloying elements chosen from Mn, Mo, Ti and Ni, each having an atomic concentration higher than or equal to 5% with a total atomic concentration of these 4 major alloying elements of less than or equal to 35%, the total atomic concentration of all of the 4 to 9 major alloying elements being higher than or equal to 80% and the remainder being made up of impurities and/or one or more minor alloying elements each having an atomic concentration of less than 5%.
 2. The alloy according to claim 1, wherein the minor elements are chosen from the list including Si, Mn, Mo, Al, Nb, H, B, C, N, O, Mg, Sc, Ti, Cu, Ni, Zn, Ga, Ge, Sr, Y, Zr, Rh, Pd, Ag, Sn, Sb, Hf, Ta, W, Pt and Au.
 3. The alloy according to claim 1, wherein the alloy contains between 0.00.5 and 0.1% atomic concentration of B as minor alloying element.
 4. The alloy according to claim 1, wherein the alloy contains between 7 and 15% atomic concentration of Ni as major alloying element.
 5. The alloy according to claim 1, wherein the alloy meets one of the following formulae expressed in atomic fractions: Al10Fe25Cr40V25, Al10Fe40Cr25V25, Al10Fe25Cr25V40, Al10Fe30Cr30V30, Al5Cr30Fe30Mo5V30, Al6Cr30Fe30Mo5V29, Al5Cr30Fe30Si5V30, Al5Cr30Fe30Mn5V30, Al13Cr25Fe25Ni12V25 Cr31Fe31V31Si7 or Fe25Cr25V25Al10Ni10Ti5.
 6. The alloy according to claim 1, wherein the alloy includes a single-phase, body-centred cubic solid solution.
 7. The alloy according to claim 1, wherein the alloy has a two-phase structure including a body-centred cubic matrix and nanoprecipitates.
 8. The alloy according to claim 1, wherein the alloy exhibits non-ferromagnetic behaviour and does not exhibit signs of corrosion after being subjected to the salt spray test according to ISO standard
 9227. 9. The alloy according to claim 1, wherein the alloy has a hardness HV10 higher than or equal to
 400. 10. An external component for horology or jewellery, wherein the component is made from an high entropy alloy with a composition containing between 4 and 9 major alloying elements chosen from the list comprising Cr, Fe, V, Al, Si, Mn, Mo, Ti and Ni with: 3 major alloying elements which are Cr, Fe and V, each having an atomic concentration comprised between 20 and 40%, 1 or 2 major alloying elements chosen from Al and Si each having an atomic concentration higher than or equal to 5% with a total concentration of these 2 major alloying elements of less than or equal to 25%, 0, 1, 2, 3 or 4 major alloying elements chosen from Mn, Mo, Ti and Ni, each having an atomic concentration higher than or equal to 5% with a total atomic concentration of these 4 major alloying elements of less than or equal to 35%.
 11. A component according to claim 10, wherein the component is chosen from the list including a case middle, a case back, a bezel, a pusher, a crown, a bracelet link, a clasp, a buckle, a prong, a dial, a hand and a how symbol. 